It would be unreasonable to demand a "proof" for God. 'Proof' suggests the kind of rigor and standards found in Mathematics and Logic. Although, there are certain arguments for God's existence that try to achieve such level of proof (Godel's Ontological Argument, Plantinga's Modal Argument etc.. ), an open minded atheist really isn't looking for a proof.

What would satisfy, would be an argument for God –

1. Whose premises are more likely to be true than false. (Not absolutely true, remember.. just needs to be more probable than its negation) 2. Isn't logically fallacious. 3. Doesn't have other explanations. (For example, the fine-tuning could be explained by the multiverse).